Atomic layer deposition (ALD) is a film forming method having an excellent property for controlling a thickness of a thin film. However, when the ALD is applied to a batch-type film forming apparatus that simultaneously forms films on plural wafers, an amount of a source gas arriving at a center portion of a wafer becomes smaller than an amount of the source gas arriving at a circumferential portion of the wafer if a surface area of a base layer, which is a film forming target on the wafer, becomes larger. As a result, the thickness of the film on each wafer becomes different in the circumferential portion and the center portion, so that in-plane uniformity of the thickness of the film is deteriorated. However, if a gas supply amount or a wafer interval is simply adjusted to improve the in-plane uniformity of the thickness of the film, quality of the film may be deteriorated and a film forming process may be complicated.